The invention relates generally to sailboats or racing yachts and more particularly to boom-footed sails.
The speed of a sailboat is related to wind velocity and direction. The mainsail of a sailboat or racing yacht is usually attached to a horizontal boom which swings about the mast with the sail as wind velocity and direction change. The function of the boom is to spread the sail outward, away from the mast. Although this may be accomplished by a sheet (line) simply pulling between the corner of the sail and the deck when the sail is trimmed close to the boat's center line, such a scheme would be ineffective when it is desired to trim the sail at large angles to the center line, as such a line would be required to end outside of the deck. Finally, a boom-footed sail allows the boat to tack with minimal crew attention required for it and is thus a convenience.
Unfortunately, the crossflow of wind through the space between the boom and the deck or cabin reduces the efficiency of the sail. This airflow produces trailing vorticies off the lower end of the sail which increases the induced drag of the sail plan, thereby reducing its propulsive force.
Attempts have been made to improve the performance of boom-footed sails by placing the boom in close proximity to the deck. However, the gain in such attempts is slight since a seal is not maintained. Moreover, the boom's necessary vertical movement would prevent a seal. Also, because the boom swings with great force, the height of the boom must for practical reasons be at some distance above the deck or cabin so that a person standing on the boat will not be hit by the swinging boom.
One solution is to extend the sail below the boom or attach an additional sail to the boom which hangs down from the boom. However this sail would be extremely difficult to operate as the boom shifts positions. In addition, in competitive racing the amount of sail surface is limited or penalized by various rating rules. Anything attached to the boom which moves with the boom would be considered additional sail area. This additional area may be a penalized or an outright violation of the rules. Such an additional sail is shown, for example, in U.S. Pat. No. 3,598,075 to Kenney which shows a two boom arrangement which provide a two surface sail shaped like an airplane wing. An additional sail or so called "fillet sheet" can be attached to the booms and stretched to the deck.
Some jibs, called "deck-sweepers", have their lower edges (feet) in contact with the deck of a yacht, thereby at least partly sealing the lower edge to the deck. Jib-like sails have sometimes been used in positions where boom-footed sails are ordinarily used, allowing a deck-sweeper configuration. In these cases, the utility of the boom is lost.